What's a name that is meaningful to you? Family name or friends name? Place name? Character in a book/film that is special to you and DP? Go for something that means something to you rather than just a name that sounds nice with Leo (unless it's completely ridiculous of course...don't call him Leo Stoke-on-Trent Smith or something!)
Alexandra - both me and my DP come from families where our middle names form a big part of our day-to-day identities so it's not so much about wanting a middle name just for the sake of having one, it's having that extension of a name if that makes sense? I appreciate that middle names might not be right for everyone and some may even think they are pointless but ultimately it is what we want for our DS so see no harm in getting suggestions from others if it helps lead us to the right one.